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Thread: Why is Sirius B a white dwarf?

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    Why is Sirius B a white dwarf?

    Sirius:
    Sirius A is, IIRC, a Type A0 star. Sirius B is much more evolved (of course) and must have been substantially larger when formed. So it must have been a mid-B at least.
    But don't such stars become neutron stars when they "die"?
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    Look Up B Sters Also

    Quote Originally Posted by Tom Mazanec View Post
    Sirius:
    Sirius A is, IIRC, a Type A0 star. Sirius B is much more evolved (of course) and must have been substantially larger when formed. So it must have been a mid-B at least.
    But don't such stars become neutron stars when they "die"?
    TRY HERE;

    https://en.wikipedia.org/wiki/Sirius#Sirius_B

  3. #3
    Depends how much mass is left at the en of its lifetime if it is under 1.4 solar masses it becomes a white dwarf. The end of the main sequence of larger stars can be extreme to say the worse it could of been a supernova or since it close to Sirius A close by it could of blown off the outer layers of Sirius B when it was a giant star. If was not near Sirius A it could of retained more mass and become a neutron star, if the sat was pointed the right it could of a pulsar to us too but that could be annoying while listening to anything on the radio or watching TV.
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    Liebert et al. The age and progenitor mass of Sirius B (2005). About 5 solar masses, which is comfortably in the zone for a carbon-oxygen WD, and about half the mass necessary for the formation of a neutron star, which is up near the bright end of the B's.

    Grant Hutchison

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    John Mendenall:
    I guess big stars loose an awful lot of mass!
    Backyard Astronomer:
    That is very interesting. Would a pulsar at ~8 LY actually interfere with radio and TV significantly?
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    Also, will Sirius become a nova and then a Supernova when A evolves off the Main Sequence?
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    Quote Originally Posted by Tom Mazanec View Post
    Also, will Sirius become a nova and then a Supernova when A evolves off the Main Sequence?
    Neither of the above. Sirius A will never get large enough to overflow its Roche lobe with the fairly distant Sirius B. So Sirius B will accrete only a small fraction of the mass blown off by A in its red giant phase. Eventually, there will be a binary pair of white dwarfs.

    Grant Hutchison

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    Thanks, Grant (a little disappointing, but what can you do?).
    Do you have an idea if a pulsar in the Sirius System would actually ruin our radio/TV?
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    Quote Originally Posted by The Backroad Astronomer View Post
    Depends how much mass is left at the en of its lifetime if it is under 1.4 solar masses it becomes a white dwarf. The end of the main sequence of larger stars can be extreme to say the worse it could of been a supernova or since it close to Sirius A close by it could of blown off the outer layers of Sirius B when it was a giant star. If was not near Sirius A it could of retained more mass and become a neutron star, if the sat was pointed the right it could of a pulsar to us too but that could be annoying while listening to anything on the radio or watching TV.
    My bold. I don't think so. A close companion does not blow away the evolving giant's envelope, but rather accretes some of the material which eventually would have been dispersed by the heat from the core in its dying gasp.

  10. #10
    Quote Originally Posted by Hornblower View Post
    My bold. I don't think so. A close companion does not blow away the evolving giant's envelope, but rather accretes some of the material which eventually would have been dispersed by the heat from the core in its dying gasp.
    The underlying point was that Sirius A could of reduce the mass of Sirius B.
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  11. #11
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    Quote Originally Posted by The Backroad Astronomer View Post
    The underlying point was that Sirius A could of reduce the mass of Sirius B.
    It is my understanding that our astrophysicists are certain that a 5-solar-mass main sequence star will get to well under 1.4 solar masses before its fusion furnace goes out, without the need for stripping by a nearby companion.

  12. #12
    Well anything under 8 solar masses would probably drop the mass needed to become a WD, I did not look up the mass of the Sirius B, I was talking general.
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  13. #13
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    Quote Originally Posted by Tom Mazanec View Post
    Do you have an idea if a pulsar in the Sirius System would actually ruin our radio/TV?
    Assuming a spherical cow...
    At and below 1GHz, pulsars are characterized by radio flux densities in the mJy to Jy range (they're very faint astronomical radio sources). Distances are on the order of a kiloparsec. Reducing distance to the order of a parsec would increase flux densities into the kJy to MJy range (comparable to bright astronomical radio sources).
    Commercial radio transmissions come in at about 1015Jy, many orders of magnitude higher.
    So I want to say no to that one, too, though of course a real cow may be insufficiently spherical.

    Grant Hutchison

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